Study population
Patients (aged at least 18 years) referred for diagnostic ICA and FFR in the Department of Cardiology, Xijing Hospital, Fourth Military Medical University between November 2020 and October 2022 due to stable or unstable angina were screened, and those who also underwent rest SPECT-MPI within 1 month were identified. Exclusion criteria included: 1) patients with acute myocardial infarctions (ST elevation or non–ST elevation), 2) iodine contrast agent allergy, 3) patients with high risk of bleeding diathesis or coagulation disorder such as malignancy, 4) patients with anemia, infectious disease or severe pulmonary disease, 5) patient with severe ventricular arrhythmias and obvious hemodynamic fluctuations. Angiographic exclusion criteria included poor angiographic image quality and severe vascular overlap. Enrolled patients were divided into 2 groups: those without hemodynamically significant CAD (defined as FFR > 0.80), or those after a successful PCI (defined as post-PCI FFR > 0.80) in patients with a significant CAD. In patients undergoing PCI, ICA and FFR used to derive Q and AMR were performed before the PCI, and the rest SPECT-MPI were performed after the successful PCI, eliminating the impact of epicardial coronary arteries.
FFR measurement
Ischemic potential of coronary stenoses was evaluated by FFR using QUANTEIN and PRESSUREWIRE (Abbott Global Health Care, USA). The instrument was zeroed three times in vitro, including the cath chamber pressure channel, aortic pressure and pressure guide wire pressure. Then, the aortic pressure and pressure guide wire pressure were equalized to the same baseline. Pressure wire was pushed through lesions and located at 2-3cm distal to the lesion. After the pressure curve waveform is stable, triphosadenine was continuously pumped through the coronary artery or vein to make the coronary micro vessels reach the maximum state of hyperemia. Finally, the pressure and FFR value were recorded.
Reconstruction of the 3D anatomy model of epicardial coronary artery
Coronary angiography images from multiple views were recorded at 15 frames per second. 3D centerlines of the targeted vessel and its large branches were reconstructed from two angiographic projections (separated by ≥ 25o) using the point-cloud based approach[16], then circular luminal contours were fitted along all vessel paths to generate the 3D anatomy models.
CFD based computational model
The pressure drop of the epicardial coronary artery can be approximately modeled by a common fluid dynamic equation[17]:
$${P}_{a}-{P}_{d}=VF\bullet Q+EL\bullet {Q}^{2}$$
1
where Pa is the mean proximal pressure, Pd is the mean distal pressure, Q is the absolute mean flow rate, VF and EL are flow resistance parameters of epicardial coronary artery, namely viscous friction (VF) and expansion loss (EL). These two flow resistance parameters (VF and EL) are mainly determined by the anatomy of epicardial coronary artery, and can be obtained by a CFD-based pressure-flow curve method from the reconstructed 3D anatomy model, which was proposed by our group previously[15, 18]. Then, with the measured Pa and Pd during routine FFR measurement, the corresponding Q can be calculated through Eq. (1). The AMR is calculated as:
$$AMR=\frac{{P}_{d}}{Q}$$
2
To provide a full understanding of those computed hemodynamic parameters, the mean flow rate and absolute microvascular resistance during rest condition were also calculated (Qrest, and AMRrest). Based on the definition of coronary flow reserve (CFR) and coronary resistive reserve ratio (RRR), the CFD based CFR (CFRCFD) and RRR (RRRCFD) were also provided.
SPECT MPI
SPECT MPI was performed using Symbia T6 (Siemens Medical Solutions, USA). In the resting state, 99mtC-MIBI 20mCi was injected intravenously, and gated SPECT MPI was performed 1 hour later by electrocardiographic R-wave trigger. 32 original images were captured at 180 degrees at 8 frames per cardiac cycle. After computer processing, the CT images of short, vertical and horizontal axis of heart and the motion images of ventricular wall were reconstructed. Experienced clinicians scored perfusion abnormalities using a 5-point scale (0-normal, 1-mild, 2-moderate, 3-severe, 4-absent) and 17-segment model. Based on the territories of the targeted vessel (LAD, LCX or RCA), the summed rest score of target vessel (SRSTV) was calculated, and patients were divided into two groups: normal MPI group (SRSTV < = 1) and abnormal MPI group (SRSTV > 1).
Statistical analysis
The IBM-SPSS Statistics 25.0 software were used for all calculations. The continuous variables were presented as mean ± SD or as medians and interquartile ranges. The categorical variables were presented as percentages. The continuous variables were compared using the t test or Mann-Whitney U test where appropriate, and categorical variables were compared using the chi-square or fisher’s exact test. Receiver operating characteristic (ROC) curve analyses were performed for discriminating normal MPI and abnormal MPI and the best cutoff values for these variables were determined by Youden’s index.